Lithography is used in the fabrication of semiconductor devices. In lithography, a light sensitive material, called a “photoresist”, coats a wafer substrate, such as silicon. The lithography tool may include a mask with a pattern including transparent and opaque regions. When the wafer and mask are illuminated, light is transmitted through the transparent regions of the mask and onto the photoresist, causing the exposed regions of the photoresist to undergo chemical reactions. The photoresist is then developed to produce a replicated pattern of the mask on the wafer.
Conventional lithography systems may include a pellicle to block particles from reaching the mask surface. A pellicle is a thin transparent layer stretched over a frame above the surface of the mask. Any particles that land on the pellicle are out of the focal plane and should not form an image on the wafer being exposed.
Extreme Ultraviolet (EUV) lithography is a promising future lithography technique. EUV light may be produced using a small, hot plasma that will efficiently radiate at a desired wavelength, e.g., in a range of approximately 11 nm to 15 nm. The plasma may be created in a vacuum chamber, typically by driving a pulsed electrical discharge through the target material or by focusing a pulsed laser beam onto the target material. The light produced by the plasma is then collected by nearby mirrors and sent downstream to the rest of the lithography tool.
EUV lithography systems use reflective masks. Conventional pellicle materials are not suitable for such systems. Currently, the strategy is to simply handle the mask in such a way to minimize the chance of particles from falling onto the mask. However, even one particle falling on a pellicle-less EUV mask may significantly affect the yield, making it very important to keep the mask surface free from defects.